Lets start at the beginning, in the beginning there was granite, loads of it, floating on the mantle like a giant presumptious sassy iceberg. Giant roots. Deeeeeeep roots. Thats granite there is over 2.5 billion years old is a Grouse Creek Block, its been thru some shit, behind it, to the east is a suture zone, or a canyon-complex where it snuggled up to the Wyoming Craton and another to the north where it completes its 3-way with the Selway Terraine, we don't know as much what this intersection looks like cause the jackwagon hotspot dumped a bunch of basalt in its guts, helping the Western Snake River Plain sink over a mile but also throwing off a lot of reconnaissance gear. Wow!
Fast forward to about 16-17myo this hot new hotspot on the block, this Yellowstone sassmouth thats been reeking havoc all across Oregon after docking as a giant island, Siletzia (54-56myo) which then got stuck and killed the giant plate which was subducting under North America, the Farallon Plate. That giant hell gumdrop got itself wedged so good it now created many of the mountains in the Western Cascades and coast range of Oregon and Washington. It vacated its tropical island paradise about 34myo as the North American plate wiggled its way over the 150mile wide inferno burning a swath thru what what was not then but is now, the High Cascades, Crooked River Caldera, the Brothers Burns Fault zone, Burns, the Steens and then finally coming up against the mega-iceberg of the continent's cratonic root, Grouse Creek.
There was a great battle.
As a wall of heat moved toward the wall and subterranean "iceberg roots" of the graniodorite craton. The tide of heat and lava rushed the granite, first at Weiser, ID, like the ocean rushes a seawall but so slow you can only see it when the top of the wave starts breaking over the seawall, our flood basalts. As most of the mass rushed back towards its volcanic past, little bits of magma flow on the shore and invade all the low lying areas of the cratons proverbial "lawn". This is how the stuff gets all the way the hell as far as places like Murphy, ID.
Imagine then, shortly thereafter, nuclear bombs start dropping, just offshore, over at Lake Owyhee, the Oregon Idaho graben. Massive killer extinction level eruptions you hear about today? Over 10 of them over the next 6millionish years (Mcdermitt, Mahogany, Three Fingers, Dinner Creek, Dooley Mtn, South Mtn, Jump Creek and so on) as the flood basalts try and try and try to push through the craton but instead just melting the shit out of it creating these rhyolites and the craton stands.
Evidence of this battle can be witnessed in amazing reality on the Banks-Lowman road, east of Garden Valley, ID. You can see the basalt try to push its way thru the tough granite of the Selway Terraine and it's like "No, go fuck you".
So millions of years of bimodal volcanic havoc, basalt, rhyolite, basalt, rhyolite, basalt, rhyolite, bimodal ya follow?